1. Field of Invention
The present invention relates to thermal head structures and more particularly to silicon semiconductor type thermal head structures.
2. Prior Art
In a heat sensitive recording system, color printed letters are printed on heat sensitive paper by exothermic dots which generate joul heat from their small resistance when current is applied selectively to them. This systems is now becoming, as is generally known, a main stay among all sorts of recording systems used in thermal instruments because of its dispensable development (owing to its peculiar primary color development), simplicity which is applicable to miniturization, low maintenance, low price etc. There are, however, three species utilized as the thermal head which are the heart of the heat sensitive recording system. The species are the thin film type, the thick film type and the semiconductor type, each having its own merits and demerits respectively. Since the present invention relates to thermal heads of the semiconductor type, advantages and drawbacks of the conventional semiconductor type will be explained.
FIG. 1 is a perspective view and outline of a conventional semiconductor type thermal head. As is seen in FIG. 1, the thermal head is designed in such a manner that a head (a) of a lengthwise rectangular strip obtained from silicon semiconductor is provided on its top surface with thermal dots (b) of similarly obtained silicon-semiconductor. The thermal dots (b) may receive from a heating circuit (not shown in the drawings) signal pulses to heat the heat sensitive paper shifting continuously over the head (a) in the direction indicated by the arrow in the picture and to effectuate momentary and selective exothermic reactions required to construct the desired printed letters. The exothermic reactions cause the corresponding spots of heat sensitive paper to develop color, thereby leading to the formation of images of printed letters thereon.
As for the advantages of the silicon-semiconductor thermal head, they are:
(i) a quick response to heat; PA1 (ii) in principle it can be made up in a fine pattern; PA1 (iii) its active elements are capable of being formed on one in the same head; and PA1 (iv) the exothermic resistance of the dot deteriorates less. PA1 (i) its production steps are intricate and large size heads are difficult to produce; PA1 (ii) its manufacturing costs are expensive; PA1 (iii) its thermal efficency is of a low degree; and PA1 (iv) it is difficult to retain heat therein.
On the other hand, the drawbacks are:
To explain more fully the reasons for the above mentioned drawbacks, the silicon head (a) shown in FIG. 1 is usually formed by means of cutting it out of bulk silicon; but since bulk silicon is limited by nature in its size, it is not possible to obtain a silicon head of as large a size as is desired. Small silicon dots (b) are hard to form at very minute intervals on the head (a) which has been obtained through a troublesome cutting process so that the manufacturing costs invariably are high. The heat capacity of the silicon itself is low from the beginning. Therefore, when heat is generated by the dots (b), it is readily absorbed by the underlying head (a). Consequently, the heat efficency is low resulting in the difficulty of retaining the heat.